DESCRIPTION: Research on visual processing in the past thirty years has revealed that neurons are usually best driven by patterns of luminance contrast rather than luminance itself. Progressing from primary visual cortex to the numerous visual areas that stretch toward the parietal and temporal lobes, optimal stimuli tend to involve more complex arrangements of luminance contrast as receptive fields increase in size. The responses of visual neurons to patterns of luminance contrast suggest that we see visual form by the pattern of activity across a great many neurons sensitive to patterns of luminance contrast. A great mystery that remains is how we see anything other than form, specifically, the surface properties of objects such as their brightness, color, and texture. A large number of psychophysical experiments have demonstrated that the perception of these surface attributes is strongly dependent on information at luminance and chromatic boundaries. However, the neural processes by which the surface properties are computed are almost entirely unknown. The research proposed in this application aims to uncover these processes by focusing on brightness perception. The long-term goal of this research is to understand the manner in which neurons in visual cortex respond and interact, giving rise to visual perception. To achieve this goal, stimuli traditionally used in psychophysical experiments are used to examine the responses of visual neurons to surface properties. The representation of brightness information will be investigated in cortical areas V1 and V2 as well as the LGN and optic tract. Recordings made at each of these sites will establish at what stage in visual processing neural activity correlated with brightness originates. Cytochrome oxidase staining will be used to determine whether brightness associated neural activity is restricted to particular compartments of V1 and V2 thought to be components of parallel processing streams. By measuring the time course of responses to various patterns of uniform illumination, the hypothesis will be tested that there are distinct fast and slow components in the cortical computation of brightness. By employing a range of visual stimuli borrowed from perceptual studies, it will be established whether there are neurons that fire in a manner correlated with brightness irrespective of the type of stimulus.